Medical rubber products are required to have high cleanability. Specifically, “Test for Rubber Closure for Aqueous Infusions” of Japanese Pharmacopoeia defines the criteria including, for example, that a predetermined amount or more of a substance such as, for example, zinc should not be detected when the product is subjected to elution with pure water. A crosslinking reaction with an added crosslinking agent is used for molding common crosslinkable rubber compositions. However, this brings unwanted detection of substances including crosslinking activators such as a crosslinking accelerator or decomposed products thereof, and decomposed products of the polymer in the elution test. Therefore, application of crosslinkable rubber compositions especially containing zinc to medical rubber products is limited a great deal.
On the other hand, not requiring chemical crosslinking, thermoplastic elastomer compositions (TPE) do not contain crosslinking agents, crosslinking accelerators and other crosslinking activators, unlike crosslinkable rubber compositions. Therefore, substances derived from these agents are not eluted. In addition, thermoplastic elastomer compositions can be molded by the same molding method used for thermoplastic resins and the resulting molded products do not need a finishing step (e.g. deburring, punching) which is needed for the molded products of crosslinkable rubber compositions, and therefore not only hygienic molding but also an economic advantage is achieved.
However, since the thermoplastic elastomer compositions do not have chemical crosslinking points, they exhibit compression set at high temperatures significantly greater than those of chemically crosslinked rubber compositions, and are disadvantageously inferior in the thermal resistance. In particular, since medical rubber products are often subjected to sterilization such as autoclave sterilization, use of thermoplastic elastomer compositions that have high compression set and are easily deformed in autoclave sterilization, as materials of medical rubber products is limited.
Dynamically-crosslinked thermoplastic elastomer compositions (TPV) with chemical crosslinking points have low compression set at high temperatures and are excellent in the thermal resistance, compared with the above-mentioned thermoplastic elastomer compositions having no chemical crosslinking points. For example, TPV dynamically crosslinked by a triazine compound has been proposed (Patent Literature 1).
However, if a dynamically-crosslinked thermoplastic elastomer composition is used for a gasket of an injection syringe, then during sterilization such as autoclave sterilization, the gasket is held at high temperatures while being closely in contact with the inner wall surface of the cylinder, under compressive strain. Therefore, the stress of the gasket to the wall surface is easily relaxed, disadvantageously resulting in lowering the pressure to the wall surface. Moreover, cooling after the sterilization makes the gasket contract to lower the pressure to the wall surface, deteriorating the sealing performance. As a result, problems such as liquid leakage are likely to occur.